James M. Nabrit III, a civil rights lawyer who fought school segregation before the Supreme Court and helped ensure that the Rev. Dr. Martin Luther King Jr.’s 1965 march from Selma to Montgomery, Ala., was allowed to go forward, died on Friday in Bethesda, Md. He was 80.

Mr. Nabrit, who worked at the defense fund from 1959 to 1989, argued 12 cases before the Supreme Court and won 9. For many years he served as the low-profile but essential second-in-charge when the group was the most persistent and prominent legal voice fighting to enforce school integration and end Jim Crow laws in the South.

“Jim was involved in many of the most important matters of the civil rights movement,” Mr. Shaw said. “The public didn’t know who he was, but civil rights lawyers knew him.”

Mr. Nabrit grew up among pillars of the civil rights movement. His father, James M. Nabrit Jr., helped Thurgood Marshall argue the cases that led to the Supreme Court’s landmark Brown v. Board of Education decision in 1954 and later became president of Howard University in Washington.

The younger Mr. Nabrit also worked with Mr. Marshall, who hired him as a lawyer with the NAACP Legal Defense and Educational Fund in 1959. Mr. Marshall, who later became the first black Supreme Court justice, founded the fund in 1957.

“When I was hired, he announced to everyone that my job title was ‘low man on the totem pole’ and that I was to be addressed as ‘boy,’ ” Mr. Nabrit recalled in a 2001 interview with the magazine The Washington Lawyer. “He always kept everyone laughing.”

Mr. Nabrit’s first assignment was to help write a Supreme Court brief arguing against an appeal of a decision that Mr. Marshall had won in Louisiana. The lower court ruling was affirmed.

In 1965, Mr. Nabrit helped write a comprehensive plan for a 50-mile march for voting rights from Selma to Montgomery that the Alabama authorities were trying to prevent. It was written to help bolster a claim by Dr. King and his associates that they had a constitutional right to conduct the march.

The plan was so elaborately detailed — noting how many marchers could participate, the route they would take and even in what farm fields they planned to sleep along the way — that The New York Times observed that the march “may take on the appearance of a biblical wandering.”

Mr. Nabrit, who wrote the plan with Jack Greenberg, the fund’s director-counsel for many years, and others, liked to joke later that it “was my only biblical writing.”

An earlier march from Selma, on March 7, 1965, ended violently when Alabama state troopers attacked civil rights supporters on the Edmund Pettus Bridge. A second attempt ended with marchers turning around after crossing the bridge. After a judge approved the plan that Mr. Nabrit had helped write, the march — which eventually included 25,000 people — went forward later that month. Congress passed the Voting Rights Act that summer.

One of the most prominent cases Mr. Nabrit worked on involved school segregation in Denver in the early 1970s. Unlike states in the South during the Jim Crow era, Colorado had no school segregation law. Instead, Denver’s school board had created segregated schools by gerrymandering the school district’s attendance zones.

When the case arrived at the Supreme Court, Mr. Nabrit began helping Gordon G. Greiner, a Denver lawyer. “The Denver case presented a different set of complications because we had to prove the cause of school segregation,” Mr. Nabrit recalled.

When the Supreme Court ruled in 1973 that the Denver school board had deliberately segregated part of the school district, it was the first time in nearly 20 years that the court had not ruled unanimously in a school desegregation case. Justices Lewis F. Powell Jr. and William H. Rehnquist dissented.

“We were very fortunate to be presenting arguments attacking the Jim Crow legal system at a time when the majority of the court wanted to do away with Jim Crow,” Mr. Nabrit recalled.

James Madison Nabrit III was born on June 11, 1932, in Houston. He grew up in Washington, where he attended segregated public schools through part of high school. He finished high school at the Mount Hermon School for Boys, now Northfield Mount Hermon, in Massachusetts.

He graduated from Bates College and Yale Law School and then worked briefly for a private law firm. He spent two years in the Army before Mr. Marshall hired him.

Mr. Nabrit’s wife of more than 50 years, Roberta Jacqueline Harlan, died in 2008. No immediate family members survive.